First check the value of session.save_path using ini_get('session.save_path') or phpinfo(). If that is non-empty, then it will show where the session files are saved. In many scenarios it is empty by default, in which case read on:
On Ubuntu or Debian machines, if session.save_path is not set, then session files are saved in /var/lib/php5.
On RHEL and CentOS systems, if session.save_path is not set, session files will be saved in /var/lib/php/session
I think that if you compile PHP from source, then when session.save_path is not set, session files will be saved in /tmp (I have not tested this myself though).
I had the same trouble finding out the correct path for sessions on a Mac. All in all, I found out that the CLI PHP has a different temporary directory than the Apache module: Apache used /var/tmp, while CLI used something like /var/folders/kf/hk_dyn7s2z9bh7y_j59cmb3m0000gn/T. But both ways, sys_get_temp_dir() got me the right path when session.save_path is empty. Using PHP 5.5.4.
The 'cli' (command line) version of php.ininormally has the same default values as the Apache2/FPM versions (at least as far as the session.save_path). You could also use a similar command to echo the web server's current PHP module settings to a webpage and use wget/curl to grab the info. There are many posts regarding phpinfo() use in this regard. But, it is quicker to just use the PHP interface or grep for it in the correct php.ini to show it's default value.
The only surefire option to find the current session.save_path value is always to check with phpinfo() in exactly the environment where you want to find out the session storage directory.
Reason: there can be all sorts of things that change session.save_path, either by overriding the php.ini value or by setting it at runtime with ini_set('session.save_path','/path/to/folder');. For example, web server management panels like ISPConfig, Plesk etc. often adapt this to give each website its own directory with session files.